Lincoln Stein discovered that Crypt::CBC fails to handle 16 bytes long initializiation vectors correctly when running in the RandomIV mode, resulting in a weaker encryption because the second part of every block will always be encrypted with zeros if the blocksize of the cipher is greater than 8 bytes.

Impact

An attacker could exploit weak ciphertext produced by Crypt::CBC to bypass certain security restrictions or to gain access to sensitive data.